Conservation of nitrogen in cattle feedlot waste with urease inhibitors.

Abstract

Feedlot cattle normally retain less than 20% of their dietary nitrogen intake. Sixty to 80% of the nitrogen excreted is normally lost through volatilization of ammonia, which is primarily generated from urea. This loss of ammonia nitrogen pollutes the environment and creates an unfavorable ratio of nitrogen to phosphorous (N:P) in the waste for crop growth. Two urease inhibitors, cyclohexylphosphoric triamide (CHPT) and N-(n-butyl) thiophosphoric triamide (NBPT) were evaluated for their ability to reduce the rate of urea hydrolysis in beef cattle feedlot pens. Initially, a total of six pens were used, two pens per treatment, with approximately 70 cattle per pen, and a single topical application of CHPT or NBPT at 20 mg/kg of manure. Essentially no urea was found in untreated pens. However, with CHPT treatment, 2 g of urea/kg of dry manure accumulated by d 4, and all gradually disappeared by d 11; NBPT conserved 3 and 3.5 g of urea/kg by d 4 and 9, respectively, and it had disappeared by d 14 (treatment [trt] x day, P = .003). A second study involved application of NBPT weekly for 6 wk. This caused urea to accumulate to a peak concentration of 17 g/kg of manure by d 30 (trt x day2, P = .001). Once the treatment was stopped the urea concentration began to decrease. When the NBPT was applied weekly, the concentration of ammonia in the waste was less for the treated pens (trt x day, P = .01), the total nitrogen was greater (trt x day, P = .04), pH tended to be lower (trt x day, P = .10), and the total volatile acids were not different (trt x day, P = .51) from untreated pens. We concluded that urease inhibitors could be used to control ammonia emissions from animal wastes, prevent environmental damage, and produce a more balanced (N:P) fertilizer from manure.